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= Invertebrate = The common fruit fly, Drosophila melanogaster, has been used extensively for research

Invertebrates are animal species that neither possess nor develop a vertebral column, derived from the notochord. By definition, this includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include insects, crabs, lobsters and their kin, snails, clams, octopuses and their kin, starfish, sea-urchins and their kin, and worms.

The most animals species are invertebrates. One estimate in the journal Science put the figure at 97%.[1] Many individual invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata[2] and some of the so-called invertebrates, such as the Chaetognatha, Hemichordata, Tunicata and Cephalochordata are more closely related to the vertebrates than to other invertebrate phyla. This makes the term "invertebrate" almost meaningless.

Etymology[edit]
The word "invertebrate" derives from a prefixed form of the Latin word vertebra. Vertebra means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate. In turn the jointed aspect of vertebra derived from the concept of turning, expressed in the root verto or vorto, to turn.[3] Coupled with the prefix in-, meaning "not" or "without",[4] the word conveys the meaning: "those that lack vertebrae".

Taxonomic significance[edit]
The term Invertebrates generates a great deal of confusion among non-biologists; it does not refer to any particular taxon in the same way that for instance Arthropoda, Vertebrata or Manidaedo. Each of those examples describes a (presumably monophyletic all valid taxon, say a phylum, subphylum or family. In referring to taxonomy of the Animalia, "vertebrata is a term of convenience, not a taxon; it has very little circumscription significance except arguably within the Chordata. The Vertebrata as a subphylum comprises such a small proportion of the Metazoa That speak of the kingdom Animalia in terms of "Vertebrates" and "Invertebrata" would be about as practical as classifying animals into mayflies and non-mayflies, or transport into rowing boats and non-rowing boats. It would be logically correct to do so, and rowing boats as such do form a practical group, but speaking of "non-rowing boats" would lump together land, sea, air and space transport in ways that rarely would be useful. In formal taxonomy of the Animalia there are higher level attributes that logically should precede the presence or absence of the vertebral column in constructing a cladogram, for example, the presence of a notochord. That would at least circumscribe the Chordata. However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry[5] or perhaps bauplan.[6] The resultant cladistic structure would not resemble anything like a binary split into vertebrates and invertebrates. At the same time, there certainly is no reason to avoid the use of the terms Invertebrata or invertebrates when they are convenient, but it is important not to confuse the terms with the names or relationships of actual taxa.

Of the million or more animal species in the world, more than 98% are invertebrates. Invertebrates don't have a skeleton of bone, either internal or external. They include hugely varied body plans. Many have fluid-filled, hydrostatic skeletons, like jellyfish or worms. Others have hard exoskeletons, outer shells like those of insects and crustaceans. The most familiar invertebrates include the Protozoa, Annelida, Echinodermata, Mollusca and Arthropoda. Arthropoda include insects, crustaceans and arachnids.

Characteristics[edit]
The trait that is common to all invertebrates is the absence of a vertebral column: this creates a distinction between invertebrates and vertebrates. The distinction is one of convenience only; it is not based on any clear biologically homologous trait, any more than the common trait of having wings functionally unites insects, bats, and birds, or than not having wings unites tortoises, snailsand sponges. Being animals, invertebrates are heterotrophs, and require sustenance in the form of the consumption of other organisms. With a few exceptions, such as the Porifera, invertebrates generally have bodies composed of differentiated tissues. There is also typically a digestive chamber with one or two openings to the exterior. Like vertebrates, most invertebrates reproduce at least partly through sexual reproduction. They produce specialized reproductive cells that undergo meiosis to produce smaller, motilespermatozoa or larger, non-motile ova.[7] These fuse to form zygotes, which develop into new individuals.[8] Others are capable of asexual reproduction, or sometimes, both methods of reproduction.

Phyla
The fossil coral Cladocora from thePliocene of Cyprus

The term invertebrates covers several phyla. One of these are the sponges (Porifera). They were long thought to have diverged from other animals early.[9] They lack the complex organization found in most other phyla.[10] Their cells are differentiated, but in most cases not organized into distinct tissues.[11] Sponges typically feed by drawing in water through pores.[12] Some speculate that sponges are not so primitive, but may instead be secondarily simplified.[13] The Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.[14] Both have distinct tissues, but they are not organized intoorgans.[15] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, they are sometimes called diploblastic.[16]

The Echinodermata are radially symmetric and exclusively marine, including starfish (Asteroidea), sea urchins, (Echinoidea), brittle stars (Ophiuroidea),sea cucumbers (Holothuroidea) and feather stars (Crinoidea).[17] Right click then go to this web for more info http://en.wikipedia.org/wiki/Invertebrate